Growing Duckweed for Backyard Aquaponics:
A Lesson in Prototyping and Reclaimed Wood
Casey Everitt firstname.lastname@example.org
Establishing an aquaponics system is an endeavor. The overhead costs alone deter many potential home farmers and the maintenance can be a hassle. The last aquaponics group concluded that the most expensive piece of a system is fish food, even over electricity.
Our group seeks to decrease the monthly cost of a home aquaponics system by growing plants suitable for consumption by both humans and fish. To do this, we will be growing duckweed.
Duckweed, a group of plants in the genus Lemna, is a tiny plant known for its tenaciousness and ability to grow in difficult conditions. It is incredibly efficient at harvesting energy, with a 4.9% conversion rate (as opposed to corn, which has a 1-2% rate). When dried, around 40% of its mass is protein. It is not widely used as a food source, but it is used in some Thai dishes and tastes a little like watercress. Tilapia, an omnivorous and widely farmed fish, can survive to maturity on duckweed alone, though additional nutritional supplements are recommended.
Our decision matrix, seen above, was constructed by out team in the early part of our project in order to decide which type of fish we would be using in our aquaponics experiment. The four fish being compared have all previously been used in aquaponics systems, so we were able to compare and contrast their traits to find which fish would be optimal for the project. The size of the fish taken into account because we did not want there to be too little room for the fish to swim around in inside the tank. The adaptability was used to measure how well the fish could respond to changing temperatures. The appetite corresponded to how easy it was to feed the fish, and the taste was a measurement of how delicious the fish could be once harvested. Though tilapia was found to be the winner in the decision matrix, we never actually used fish in our project as we eventually scrapped the idea of making an entire aquaponics system because we wanted to focus on the growing of the food instead. However, we had intended to use one or two fish in order use their feces as a nutrient source for the duckweed.
Left: A map of Africa highlighting the areas most suitable for tilapia farming. Right: A map showing the native range of duckweed, all species.
Our proposed system could be implemented around the world — with some constraints. We would like to avoid introducting non-native species of duckweed into an area if possible, since it spreads almost virulently if left unchecked. According to the April 2017 ISCDRA Duckweed newsletter (yes, really!), duckweed transfers ponds by sticking to the feathers of migratory waterfowl, such as ducks and geese. Duckweed can be spread up to 250 kilometers in this manner.
Another constraint is the weather. Tilapia prefer warmer temperatures, and when all available electricity goes into keeping the water oxygenated, heating can be an issue. Tilapia farming is usually restricted to areas where it doesn’t go below sixty for long stretches of time; in these places, tilapia is substituted for bass.
The social implications of tilapia farming must also be considered. Tilapia are native to Africa and the Middle East, and people elsewhere will be unused to eating them. Most appropriate technology startups fail beause they do not consider how people will react to their inventions: in the case of our setup, giving a tilapia farm to someone who doesn’t like tilapia is not the ideal situation.
Additionally, our setup specifically targets urban areas. The tiered a-frame is designed for areas with little sunlight and not much space. It would best belong in a community garden or similar. Dr. Ryan Alaniz, after his talk with our class, suggested that we contact elementary schools in low-income California neighborhoods to see if they would be interested in establishing a life sciences program using our product. We did contact a few schools, specifically in the town of Guadalupe, CA as this is a less fortunate and less wealthy area than the rest of the county that it resides in. The intent for this outreach effort was to try and show the local science programs how aquaponics works in the hope that a school would start a similar project that could potentially spark an interest in their students for math and science type subjects. Though this effort was made, the search for an adopter is ongoing as no response was ever received. However this is not a complete loss as we wanted to make sure it worked before we initiated any educational collaboration.
Materials and Construction:
This project cost us around $35.
The expenses were split fairly evently between all of the purchaced materials, however it should be noted that we didn’t invest in fish or in duckweed cultures.
In choosing our materials, we kept in mind two ways we could improve the project: “How could we make this better?” and “How could we make this cheaper?” Our goal was to stay in the middle of these two points.
To make our setup better, we could have used a metal frame instead of a wooden one. We could have used flat-bottomed boxes instead of ladder-plank bottoms. We could have bought a kiddie pool or tank of equivalent size, instead of digging a hole in the ground. But if we had the budget for those things, we wouldn’t be building this at all.
To make our setup cheaper, we could have used branches instead of planks. We could have lashed our wood together instead of using screws. We could have used nets instead of boxes (this may have been better, actually).
We saved money and cut corners however we could, keeping in mind our target audience of low-income schools and communities. This proved to be our downfall, as one of our scavenged pieces of wood didn’t hold up under pressure.
Actually building the thing was significantly harder than expected. Usually only one person was available to work at a time, so progress was slow. We had to stick to a very clear timeline that didn’t allow for testing in between stages. Since we couldn’t transport it after it was fully built, we had to move it to the SEF in pieces. And since we couldn’t test it until it was upright, we were unable to forsee easily avoidable problems. When the whole thing eventually collased under its own weight, we weren’t even surprised.
Tom Kelly’s guideline to a successful project is to “Fail early, fail fast, and fail often.” We did none of those things. Instead, we waited until the last possible day to diagnose our project with irreparable damage. Due to conflicting schedules, the three of us were never able to meet outside of class, and since one of us usually had a necessary material or tool, little work got done outside of the three hours on Monday alloted for lab time. Since our project had a very specific timeline, we were only able to transport it to the SEF last weekend and we were only able to test it today. We were plagued by issues even before we filled the thing with water: Our drill’s batteries stopped working, so we had to rely on screwdrivers, resulting in splitting wood and stripped phillips-heads. The pond we dug was not level, so we had to prop up one end of the structure with scrap wood. Furthermore, our supply of duckweed had been taken over by another plant more suited to the changing spring weather.
Still, it’s not a complete loss. A failure is just a lesson learned for next time. The main lesson we learned was that even when it looks like you’re being given a lucky break, cutting corners rarely pays off. Make sure the wood is sturdy before you screw it into place. Ask an engineer before spending too long on a project you’re not sure will hold up. To quote our lab tech, Toby, “We should have used 2x4s… famous last words.”
So, what exactly went wrong?
We decided to use reclaimed wood for the sides of the water boxes. One of the pieces of wood had several knots in it, which resulted in multiple holes going straight through the plank. This would not have been a problem if we kept the wood in its original form, but since we cut it into smaller pieces, the stress on those points was significantly greater.
Additionally, we did not account for the fact that the weight of water (or any mass) gets exponentially bigger when its volume increases. This would explain why our smaller tests showed positive results when the larger one was so catastrophic.
From Pete: Thanks for the creativity. It might have been a good idea to look into who if anyone has done anything like this. Additionally, do you have any advice/outlook for the next group?